Process for forming permeable sheet material



United States Patent PROCESS FOR FORMING PERMEABLE SHEET MATERIAL VerneL. Simril, Williamsville, N. Y., assignor to E. I. du Pont de Nemoursand Company, Wilmington, Del., a corporation of Delaware No Drawing.Application November 4, 1952, Serial No. 318,732

6 Claims. (Cl. 117-7) This invention relates to synthetic leather and,more specifically, to a process for forming a synthetic leather havingthe desirable inherent physical characteristics of leather.

Patents relating to methods and techniques of preparing varioussynthetic leather compositions have been granted to inventors as farback as 1850'; and even before that time, a great number of attemptshave been made to devise a single and rapid technique of producingsynthetic leather compositions. In most of the early art, pyroxylin wasused to coat or impregnate various types of fibrous base materials toprepare leather substitutes. As time went on, pyroxylin/oil/pigmentcompositions were widely used as coating or impregnating compositionsfor various woven or non-woven fibrous base materials.

In the early stages of the synthetic leather industry, the mainobjective was to simulate the general appearance of leather. In todaysmarkets,.coated fabrics, particularly the vinyl coated fabrics, areoutstanding as leather substitutes in such applications as handbags,bookbindings, brief cases, card table covers, luggage, etc. In suchapplications, the coated fabrics are satisfactory because the generalappearance of leather is simulated and the coated fabrics possess someof the desirable properties of leather. However, as compared to leather,the coated fabrics lack the properties of good tear strength, softness,and the ability to breathe or transpire water vapor and air, which arecharacteristic of leather; and, although the coated fabrics are usedinsuch applications as chair coverings, much is left to be desired,especially with respect to water vapor and air permeability. Up to thepresent time, synthetic leather compositions have made little or noinroads into the boots, shoes, and glove markets, mainly because oftheir inability to breathe, in addition to lack of good tear strengthand softness. As used herein, the term breathe means transpire watervapor and air.

In general, the use of a synthetic leather composition in boots, shoes,gloves, etc., is mainly dependent upon its ability to breathe (usuallyexpressed in terms of water vapor permeability). Physical tests on thewater vapor permeability of leather indicate that leather transpireswater vapor about /a as readily as free air. In general terms, shoeupper leather samples having a thickness of 0.0l6"0.104" have a watervapor permeability within the range of LOGO-18,000 grams/ 100 sq.meters/hr., when tested according to the method of Kanagy and Vickers,Journal of American Leather Chemists Association, 45, 211-242 (April1950), in an atmosphere of 23 C. and 90% relative humidity. Hereinafter,the ability of synthetic leather compositions to transpire Water vaporwill be expressed in terms of leather permeability (LPV), as measured bythe above test, in grams/ 100 sq. meters/hr. Based upon comfort testswhich have been carried out, under the above conditions, the minimumtolerable leather permeability for shoe upper leather is about],6002,000 grams/ 100 sq. meters/hr. Preferably, for shoe upper leather,the leather permeability should be 4,00020,000. Actually, there is noupper limit, but the composition must be substantially resistant topenetration by liquid water and have acceptable strength properties.

An object of the present invention is to provide a synthetic leatherhaving outstanding breathing qualities. A further object of the presentinvention is to provide a process of preparing a synthetic leatherhaving the requisite properties for fabricating boots, shoes, gloves,chair coverings, and other articles wherein a composition capable ofbreathing is required. A still further object is to provide a process ofpreparing a synthetic leather having a tenacity, flex life, elongation,tear strength, modulus and leather permeability equal or superior to thevarious types of genuine leather. A still further object is to provide aprocess of preparing a synthetic leather in which the aforementionedphysical properties may be tailored to the desired end use. Otherobjects will be apparent from the description given hereinafter.

These objects are realized by the present invention which, brieflystated, comprises forming a compact, essentially watervapor-impermeable, and continuous composite sheet by hot pressing acomposition comprising essentially a relatively non-extensible material,either in the form of fibers and/or discrete particles, thoroughlyimpregnated with a relatively extensible, substantially non-adherent,binder material, and thereafter stretching said sheet within the limitshereinafter set forth, followed by relaxing, to produce a sheet havingoutstanding water vapor and air permeability.

The expression relatively extensible material, as used herein, meansthat the material must be stretchable under the stretching forces to beemployed and must elongate during the stretching and substantiallyrecover upon releasing the stretching forces. The expression relativelynon-extensible material means that the material stretches to asubstantially lesser extent, if at all, than the relatively extensiblematerial under the stretching forces imposed.

The expression substantially water vapor-impermeable as applied to theinitial composite sheet (i. e., before stretching) means that the sheethas a leather permeability value (LPV) of less than 1,000 grams/ 100 sq.meters/hr.,

. measured at 23 C. and 81% relative humidity. In general terms whichmay be applied to any combination of relatively extensible andrelatively non-extensible materials useful in the present process, thewater vapor permeability of the initial composite base sheet issubstantially no greater than that of homogeneous sheet of therelatively extensible material or the relatively non-extensiblematerial, whichever has the higher water vapor permeability. Usually,providing the initial composite base sheet is prepared in such a mannerthat there are substantially no voids present in the sheet, the watervapor permeability of such a base sheet is intermediate between thepermeabilities of homogeneous sheets of the two major components of thebase sheet. Actually, the leather permeability values (LPV) of theinitial compacted substantially water vapor-impermeable sheets which aretransformed into permeable sheets by following the present process arelow not only because the sheet is compacted by hot pressing, but alsobecause of the thickness of the sheets which are useful for convertingto synthetic leather sheets. That is, the initial sheets are usuallybetween 0.015"0.05 in thickness, and homogeneous sheets of either thebinder polymer or the structural fiber having thicknesses within thisrange also have low LPVs. Normally the LPV is appreciably less than1,000, and in no case greater.

The present invention resides in the discovery that stretching asubstantially water vapor-impermeable composite sheet comprising anextensible or stretchable material and a relatively non-extensible ornon-stretchable material results in the formation of a sheet which ispermeable to water vapor and air. Generally, the degree of permeabilityimparted to the stretched sheet essentially depends upon the particularrelatively non-extensible material employed, e. g., particulate orelongated (fibers); the particular relatively extensible materialemployed; the ratio of extensible and relatively non-extensiblematerials in the initial sheet; the extent of stretch (either in onedirection or two directions); and the degree of adhesion between the twomajor components of the sheet. Other specific factors which affect notonly the resulting permeability but also other physical properties ofthe resulting sheet will be discussed further hereinafter. It isbelieved that the stretched sheets are vapor-permeable because theinternal structure of the initial sheet is modified. Stretching theinitial sheet appears to pull the extensible component away from therelatively non-extensible component, and this results in the formationof voids or capillaries around the relatively non-extensible component.Hence, the resulting stretched sheet has a crosssectional structureessentially consisting of a binder material reinforced With therelatively non-extensible particles or fibers, the areas immediatelyadjacent the particles or fibers being voids. These voided areas form astructural pattern substantially identical to the distribution of therelatively non-extensible material throughout the extensible bindermaterial. Preferably, the relatively nonextensible material is in theform of a fibrous mat, the fibers being intertangled. The use of thistype of fibrous mat as the relatively non-extensible material results inthe formation of interconnecting voids or capillaries throughout thestretched sheet.

The total volume of voids formed in the stretched and relaxed sheetdepends not only on the quantity of structural fibers present in theinitial, i. e., before stretching, sheet but also upon the degree ofstretch and the amount of recovery of the binder polymer upon relaxing.Normally, the binder polymer after stretching does not recovercompletely, i. e., 100%, especially after being stretched as much as40-50% in one or both directions. Incomplete recovery is evidenced by anincrease in the thickness of the sheet after stretching and relaxing.However, so long as the adhesive bonds between fiber and binder arebroken in stretching, it may be desirable to obtain almost completerecovery or a high degree of recovery because the resulting sheet wouldbe softer and yet highly permeable because of the internal surface areaformed by breaking the adhesive bonds between binder polymer andstructural fibers.

The relatively extensible material may be selected from the large classof soft, thermoplastic polymers and, more specifically, from the classof polymers which may be generally classified as elastomers orelastomeric materials, as set forth by H. L. Fisher (Industrial andEngineering Chemistry, Aug. 1939, page 942). Those polymers which areonly partially elastic or elastomeric and are not strictly classified aselastomers include ethylene polymers such as polyethylene, chlorinatedpolyethylene; vinylidene chloride/acrylonitrile copolymers; variouspolyamides such as N-methoxymethyl polyhexamethylene adipamide; andcopolyesters made from ethylene glycol, terephthalic acid and sebacicacid of the general types described and claimed in copendingapplications U. S. Serial Nos. 150,811 and 150,812, filed March 20,1950, in the name of M. D. Snyder, and now Patents 2,623,033 and2,623,031, respectively polyvinyl acet'als such as polyvinyl butyral andpolyvinyl laural; ethylene/vinyl acetate copolymers in which the ratioof ethylene to vinyl acetate ranges from 1.411 to 11:1. Polymersgenerally classified as elastomers include plasticized polyvinylchloride; natural rubbers; synthetic rubbers such as neoprene (poly-2-chloro-1,3-butadiene polymer), chloro-sulfonated polyethylenebutadiene/acrylonitrile copolymers, and other butadiene copolymers. Allof the foregoing may be employed with or without plasticizers.

With respect to the binder polymer, it is to be understood that the termthermoplastic is meant' to include those polymeric materials which areat least initially thermoplastic under the conditions of hot pressing.in other words, the term initially thermoplastic includes bindermaterials which melt and flow under the conditions of hot pressing; andat higher temperatures, these materials, e. g., rubber and syntheticrubber compositions, may cure and be converted to a substantiallyt'hermoset condition.

The relatively non-extensible material may be selected from a widevariety of materials in the form of particles of various shapes or inthe form of elongated shapes such as fibers. The use of relativelynon-extensible materials in fiber form is preferred. This is because itis desired to obtain interconnecting voids or capillaries in theresulting stretched sheets. Since the structural fibers are serving toreinforce the binder material and provide for the formation ofinterconnecting capillaries or pores having substantially the sameinterconnecting network pat' tern as that of the structural fibers,selection of fibers of a particular length and denier must be consideredwith these two functions in mind. For the purpose of providing adequatetensile strength, tear strength and flex life, the structural fibersshould be at least about 0.5" in length. As a general observation, thestrength properties of the resulting compositions do not increaseappreciably when using structural fibers greater than about 1.5" inlength. On the other hand, from the standpoint of handling non-wovenmats of fibers on standard textile machines, it may be more convenientto use longer fibers, for example, as long as 8 or longer. With respectto the network of interconnecting pores formed throughout thecross-section of a sheet treated in accordance with this invention, theuse of very short fibers, e. g., 0.01" flock, does not produce acomposition having optimum permeability. When the length of structuralfibers is increased from 0.25 to 0.5", the permeability appears toincrease; but no appreciable increase is obtained when fibers longerthan 0.5" are employed.

It is within the scope of the present invention to employ fibers havinga denier within a relatively wide range. Normally, textile fibers havinga denier Within the range from 1-3 denier/filament are employed. On theother hand, fibers having a denier as low as 5.5 l0 denicr/ filamenthave been used in conjunction with fibers of greater denier, e. g., 1-3denier/filament. Usually, it is entirely practical to employ very thindenier fibers so long as the cohesive bonds within the fibers aregreater than the adhesion between fiber and hinder, or, in other words,so long as the fibers are distinguishable as such in the fiber/bindercomposite. it is especially advantageous to position very thin denierfibers at the surface of the present compositions in order to producesheeting which will retain its original surface appearance afterrepeated abrasions. An extreme condition with respect to employingfibers of higher denier is the use of fibers substantially greater thanabout 16 denier/filament, these fibers being relatively stiff andbristle-like.

It is to be understood that oriented or unoriented fibers or mixturesthereof may be employed as the structural fibers in the presentcompositions. The use of orientable, but unoriented, textile fibers ishighly advantageous in preparing compositions of high tear strength andimproved pliability and flex life. Furthermore, it is within the scopeof the present invention to employ mixtures of fibers of various sizes,i. e., length and denier, for the purpose of producing compositionswhich retain their original surface after repeated abrasions. This isaccomplished by using short fibers, i. e., less than /2", in combinationwith the longer fibers in the structure. The chemical composition of theshort fibers may be the same or difierent from that of the longerfibers. Usually, the short fibers are strategically concentrated nearthe surface of the initial structure in order to reduce fuzzing of thesurface resulting from abrasion. On the other hand, the short fibers maybe uniformly dispersed throughout the binder with the longer fibers.Non-extensible materials in the form of particles include pigments suchas zinc oxide, talc, clay, diatomaceous earths, and various polymericmaterials in particulate form such as polyamides, polyethyleneterephthalate, polytetrafiuoroethylene, etc.

It is to be understood that the non-woven fibrous mats employed inpreparing the polymer-impregnated, substantially impermeable, initialsheets may be fabricated in accordance with any well known batch-wise orcontinuous technique such as by carding machines, air depositionapparatus, and water deposition or paper-making techniques. Furthermore,the resulting fibrous mats may have their component fibers orientedsubstantially in one direction or randomly arranged. Individual matshaving the fibers oriented in one direction may be cross laminated. Inany event, the fibers must be interconnecting so that the resultingcapillaries or void spaces are interconnecting.

In selecting the components, i. e., the extensible and relativelynon-extensible materials, to be combined in preparing the initialcomposite sheet, two main factors must be taken into consideration: (1)the extensibility of the two components must be different; therelatively non-extensible material must be relatively non-extensible ascompared with the extensible material under the stretching forces(expressed in terms of per cent elongation) to which the sheet is to besubjected. For example, if a polyamide fiber is employed as therelatively non-extensible material, the selected binder or extensiblematerial must be one which can be elongated under forces which stretchthe fibers to a substantially lesser extent, if at all. Generallyspeaking, a polyamide fiber is not considered to be a non-extensiblematerial; but in accordance with the definition employed herein, therelativelynon-extensible material is defined with respect to or inrelation to the extensible material or binder employed in combinationtherewith. (2) Adhesion between the extensible and relativelynon-extensible material should be such that the adhesive bonds can bereadily broken by stretching. The preferred or ideal case is one inwhich there is substantially no adhesion between the relativelyextensible and relatively non-extensible material. With composite sheetsfabricated from such basic components, only moderate stretchingforcesare required to break the adhesive bonds and thereby form voids withinthe stretched sheet. On the other hand, it follows that initialcomposite sheets composed of a binder polymer and a fiber which formstrong adhesive bonds must be subjected to drastic or extensivestretching in order to rupture these bonds for the purpose of producingvoids within the stretched sheet. This is not generally practical fromthe standpoint of the excessive forces required. Hence, it is preferredthat the degree of adhesion between the extensible and relativelynon-extensible materials be as low or insignificant as possible.

The ratio of relatively extensible material to relatively non-extensiblematerial in the initial sheet may vary from 30:70 to 70:30. However, theoptimum quantity of extensible material in the initial composite sheetranges from 40-60%, based upon the total weight of the two majorcomponents. Normally, the densities of the extensible material and therelatively non-extensible material are relatively close; and in suchcases, the ratio of the two components may be expressed on either avolume or weight basis. Generally, as the quantity of extensible orbinder material in the compositions is reduced, the initial compositesheet becomes more and more permeable to vapor (the stretched sheet ishighly permeable); but other properties and characteristics of thematerial (the stretched sheet) are not satisfactory for use as a leatherreplacement, for example, in boots, shoes, gloves, chair coverings, etc.Stretched compositions which contain too low a quantity of extensiblematerial are extremely fuzzy at the surface, and the abrasion resistanceis very poor. The general feel is more like that of felt instead ofleather, and the tensile strength and tear strength are below thetensile and tear strength of materials having from 40-60% of theextensible material. This is because in most cases the amount ofextensible or binder material is insutficient to hold the relativelynon-extensible material, e. g., fibers, together. On the other hand, asthe amount of extensible or binder material is increased, the initialcomposite sheet has properties approaching those of a homogeneous filmor sheet of the extensible or binder material. For example, theelongation and modulus increase as the extensible or binder content isincreased; and the initial vapor permeability is very low (the sheetsare substantially impermeable) as is the permeability of homogeneousfilms of the extensible or binder polymers employed. Furthermore, thetear strength also falls off rapidly at high binder content because thefilm is tearing essentially as a homogeneous (non-reinforced) film. Allthese factors are also true for corresponding stretched sheets. Thecontent of the binder or extensible material may also be varieddepending upon the particular combination of extensible and relativelynon-extensible materials in the initial composite sheet. As previouslystated, those components which, when combined into the initial sheet,form strong adhesive bonds, offer greater resistance to stretching andusually require greater amounts of stretch to obtain the desired vaporpermeability. In such cases, the content of extensible or bindermaterial in the initial composition may be. reduced somewhat in order tolessen the forces required for stretching. It is to be understood,however, that certain compromises must be made with respect to thosephysical properties mainly affected by reducing the content ofextensible or binder material. In addition to the major substituents,the initial sheet may contain minor proportions of various additivessuch as plasticizers, dyes, etc., plasticizers usually beingincorporated with the extensible material.

The improvement in the water vapor permeability of the initial compositesheets effected by stretching is directly related to the amount ofelongation. At very low elongation, i. e., 510%, the sheets remainsubstantially impermeable even though they are stretched in twodirections. However, as the amount of elongation is increased, the watervapor permeability increases; and the improvement is outstanding whenthe sheets are stretched in two directions. Generally, the nature of theextensible and relatively non-extensible material employed in theinitial composition determines the amount of stretch required to effectthe desired improvement in water vapor permeability. However, in allcases, stretching in two directions, that is, biaxially, appears toproduce the greatest improvement in water vapor permeability for a givenextent of stretch or elongation. Furthermore, it has been generallyobserved that extremely high elongation, i. e., 50% and above, althoughimparting excellent porosity or breathing qualities, usually degradesthe other physical properties necessary to produce a good syntheticleather. Specifically, the tensile strength decreases appreciably athigh elongation. In general, as the extensible material or binderpolymer is extended to the limit of its extensibility, the binderruptures; and from this point, in the case of a fiber-reinforcedcomposition, only fiber-fiber separation is measured. Preferably, theinitial sheet should be stretched biaxially from 10-40%, the degree ofstretching being substantially the same in each direction.

Since the tensile strength of the present synthetic leather compositionsdecreases as the extent of stretch increases, the tensile modulus alsodecreases. In general, a low modulus indicates a soft material; and thisis usually desirable in a synthetic leather composition, es pecially foruse in boots, shoes, gloves, etc. However, the degree of softness whichmay be imparted to these compositions by stretching is limited by thedecrease in tensile strength which may be tolerated. From the foregoing,it is manifest that the present process is highly flexible; and theresulting synthetic leather compositions may be tailored to the desiredend use by obtaining a balance between the desired vapor permeabilityand other physical properties, such as tensile strength, tear strengthand tensile modulus.

Any desired expedient for stretching the initial sheet may be employed.Where the degree of adhesion between the extensible and relativelynon-extensible material is high, drastic conditions of stretching may berequired. So-called drastic conditions of stretching usually meanssoaking or conditioning the initial composite sheet in a hot liquidfollowed by stretching in one or both directions while the sheet is atan elevated temperature. On the other hand, mild stretching may becarried out by hand working, i. e., crumpling and/or flexing a sheet.Working the sheet by hand or mechanical means, however, does not alwayslead to producing uniform permeability throughout the entire sheet. Inmost cases, uniform permeability is only obtained by elongating thesheet in one direction and then in the opposite direction, or just inone direction, or by stretching the sheet simultaneously in bothdirections on a tentering frame.

As set forth hereinabove, in order to obtain synthetic leathercompositions having outstanding breathing qualities and strengthproperties for use in fabricating boots, shoes, gloves, chair coverings,etc, by following the process of the present invention, the initialcomposite sheet, i. e., before stretching, must be substantially watervapor-impermeable. Initial composite sheets which are substantiallypermeable to water vapor, i. e., have an LPV substantially greater than1,000 grams/ 100 sq. meters/hr. (measured at 23 C. and 8l% relativehumidity), are not normally convertible into a synthetic leathercomposition of satisfactory strength properties by following the presentprocess. Initial composite sheets having a content of relativelyextensible or binder polymer substantially less than 30% are consideredto be within this classification; that is, the water vapor permeabilityof the initial sheet is high because the content of extensible or bindermaterial is too low.

In the preparation of the initial sheet, any technique which willprovide a composite sheet having particles or fibers of the relativelynon-extensible material distributed uniformly throughout the relativelyextensible material may be employed. Usually, in order to prepare auniformly composited sheet, it is necessary to compact the sheet bysubjecting it to heat and pressure. Pressures in the neighborhood of 500p. s. i. appear to be satisfactory, but any combination of pressure andelevated temperature which causes the relatively extensible or bindermaterial to flow is satisfactory. The pressing temperature is usuallyabove the flow temperature of the selected extensible or binder polymer.After the pressing or compositing step, the initial base sheet issubstantially free of voids and substantially impermeable to air.Furthermore, the water vapor permeability is low and it is notsubstantially greater than the water vapor permeability of a homogeneoussheet of either the binder material or the reinforcing material. Inpreferred form, a mat of intertangled fibers (the relativelynon-extensible material) is thoroughly impregnated with the extensibleor hinder material so that each individual fiber is surrounded by theimpregnant. Furthermore, it is important that the fibers protrudethrough the surfaces of the sheet to provide for entrances and exits forthe passage of vapor through the stretched sheet, the internal structureof the stretched sheet comprising a network of interconnecting pores orcapillaries. Various other methods of fabricating the initial compositesheets of the present invention are as follows:

(1) A fibrous mat may be pressed into a film or sheet of the extensibleor binder polymer. The pressing temperature must be above the flowtemperature of the polymer. In some applications, one sheet of thebinder polymer-may be used, or the fibrous mat may be inserted betweenadjacent sheets.

(2) A fibrous mat may be impregnated by passing it through a solventsolution of the extensible or binder material. The solvent may beevolved by well known techniques.

(3) The binder or extensible material in the form of a powder may bedistributed upon the surface of a moving fibrous mat, and thereafter themat is subjected to a source of heat to melt the polymer and impregnatethe mat. Various sources of heat may be employed, for example,infra-red, dielectric heat, heated rolls, hot plates, air oven, etc.This operation is followed by passing the sheet through pressing rollsto form a uniformly flat sheet.

(4) A moving fibrous web may be fed into the nip of a set of calendarrolls upon which the binder polymer is plasticated. The binder polymerand mat are fed between the nip, and the pressure of the rolls serves toimpregnate the mat.

(5) A dispersion of particles or fibers (or a mixture thereof) of thebinder or extensible material and particles or fibers of a relativelynon-extensible material is prepared in aqueous medium or other inertliquid medium. This dispersion may be coagulated continuously to form asheet consisting of a continuous matrix of the hinder or extensiblematerial in which particles or fibers of the relatively non-extensiblematerial are randomly dispersed throughout. The resulting sheet may becomposited by subjecting it to light pressure by feeding between the nipof a pair of heated rolls.

(6) A fibrous mat may be passed through a liquid polymerizable organiccompound to impregnate the mat with the polymerizable liquid.Thereafter, the impregnated mat is conducted through an oven or under abank of infra-red lights to bring about polymerization of the liquidcomponent. In such a process, the liquid polymerizable organic compoundshould be thickened to a suitable viscosity, probably by dissolving asmall amount of the hinder or extensible polymeric material in the corresponding liquid monomer.

(7) A continuously moving fibrous mat may be passed under aflamespraying apparatus which sprays uniformly the molten extensible orbinder material onto one or both surfaces of the fibrous mat. This typeof apparatus would have to be set up in such a way as to provide forcomplete impregnation of the mat.

(8) A hinder or extensible polymeric material in the form of a granularpowder may be distributed onto a hot moving belt in order to sinter theparticles together to form a continuous sheet. This sheet of binder orextensible polymer along with a fibrous mat may be fed into the nip of aset of pressing rolls to impregnate the fibrous mat with the binder orextensible polymer.

(9) A solvent solution or dispersion of the binder or extensible polymermay be continuously sprayed upon one or both the surfaces of a movingfibrous mat. The resulting impregnated mat may be passed thereafter intothe bite of heated pressure rolls to compact the sheet.

(10) A mixture of fibers of extensible and relatively non-extensiblematerials is carded, and the resulting mat is fed into heated pressurerolls to melt the extensible material to form a continuous matrix of theextensible material having fibers of the relatively non-extensiblematerial distributed uniformly throughout.

In forming the initial composite sheets of the present invention, it isto be understood that the binder or extensible material must melt orflow at a substantially lower temperature, usually at least 20 C. lowerthan the relatively non-extensible material in order to provide forcompositing or compacting the combination of materials to form theinitial sheet.

The following examples will serve to further illustrate the preparationand nature of the synthetic leather compositions of the presentinvention.

The compositions tabulated and. specified in Table 1 were prepared inaccordance with the following general procedure: Crimped staple fibersof polyhexamethylene adipamide, 2 /2" long and 3 denier/filament, werecarded to form non-woven mats or webs. The web was cut into sections andthe various sections were put between screens and immersed into anaqueous solution of wetting agents containing 2% octyl sodiumsulfosuccinate and 2% of a pared in the manner described above exceptExample 17 which was prepared by impregnating the non-woven mat with abenzene solution of polyisobutylene. After impregnation, the solvent wasevolved and the impregnated mat was subjected to heat and pressure asmentioned above.

The resulting substantially water vapor-impermeable composite sheetswere then placed in a stretching apparatus and elongated a definitefraction of their original length or length and width, e. g., one-way ortwo-way stretching. The stretched sheets weer then subjected to variousphysical measurements, i. e., tenacity, elongation, modulus, tearstrength and leather permeability (LPV). In all examples containedherein, the value of LPV was measured at 23 C. and 81% relativehumidity.

TABLE 1 Percent Unit Tinius Stretch- Percent Weight of Thick- Elonga-Olsen L. P. V., Example Extensible (Binder) Material 1D=one Exten-Initial ness Tenacity tron Modulus Tongue gms./100 sq.

direction; sible Sheet (lnches) (p. s. 1.) (percent) (p. s. r.) Tearmeters/hr. 2D=two Material (gms./sq. (gms) directions meter) 1plasticized neoprene 40% 1D 53.5 713 MD 4,122 127 9, 756 16, 500 4, 375

2 TD 1,079 148 705 2 do 2D 59 8 46 MD 6, 660 115.7 5,892 19,100 3,020

' TD 2, 076 170. 7 2,222 3 -do 0 57 791 MD 5,824 107 13,478 16, 000167.5

' i TD 2, 176 183 10, 339 4-; butadiene/styrene copolymer 10% 2D 51.2654 .032 4, 734 107.8 8,870 10,683 3, 020 5 neoprene 2D 40. 5 958 .0843, 200 90 5, 300 28, 500 11,000 d 40% 2D 50. 5 532 033 4, 900 95 6, 65010, 000 5, 500 0 629 023 3, 700 142 23, 000 16, 300 25 0 63 85,7 .035 MD5, 900 82 16,486 ,000 30 TD 3,004 130 13,182 16,000 d0 40% 2D 1, 151.059 3, 83 4, 415 12, 000 1, 800 do 30% 2D 1 47.6 85,0 .037 MD 5, 255 8010,200 8, 590 1, 436 TD 1, 931 138 6, 037 12, 740

neoprene BAG 0 53.5 733 .030 MD 4, 605 97 14, 631 9, 700 30 TD 496 1,277. 559 10,260 d0 1--------T----,- 30%.21) 52.5 709 .046 MD 2, 073 56.54,117 7, 510 5,000 TD 360 63. 75 2, 464

50 neoprene 700/50 neoprene BAG 0 56. 2 741 026 1, 589 161 8, 595 12,000 40 do 30% 2D 58 840 040 MD 5, 152 97. 5 7, 066 11, 000 1, 100

TD 1, 025 174 2, 489 16, 000 50 methyl Ccllosolve acrylate/ 50 vinylacetate 30% 2D 54.6 708 .039 MD 4,185 123 3,112 11, 000 1, 205

TD 1, 292 191 1, 052 do 0 54. 4 777 .031 MD 5, 200 108 1, 598 11,000 375TD 1,379 265 2,734 polyrsobutylena 30% 2D 827 060 MD 3, 597 112 2,09512,500 4, 568

, TD 1 116 171 3,000 do 0 65 854 .040 MD 5,800 121 6,440 9, 500 45 TD 1880 187 4,560 methyl acrylate., 40% 2D 55. 3 819 034 MD 7, 687 83 13,100 6, 300 1, 633

TD 3, 924 114 12,000 10 30% 2D 53 625 .027 MD 6,176 79 11, 000 4,500 1,433

TD 2, 801 89 12,000 d0 0 53 701 .027 MD 6, 900 66. 5 22, 965 5, 500 151TD 4,000 96 ,500 50 neoprene/50 butyl 1111213612-- 40% 56 1, 390 089 2,400 150 2, 600 6, 500 8, 600 50 methyl Cellosolve methacrylate/50methy1acrylate.. 40%; 62 987 .046 4, 000 108 4, 700 11, 000 2, 650 .,do0 64 851. 034 5, 100 112 14, 500 10, 500 485 neoprene with stifieningagent. 40% 62 867 035 5, 090 87 10, 700 8. 600 2, 200 d0 0 63. 5 1, 113041 4, 800 98 21,400 12,000 629 1 MD =machine direction (longitudinal).

3 TD =transverse direction.

sodium salt of alkyl benzene sulfonate. The webs were then squeezedthrough a two-roll wringer and allowed to dry. A dispersion or latex ofthe various extensible or hinder polymers indicated in Table 1 wasprepared by various known techniques. Many of the latices arecommercially available. After the webs were dried, they were thenimmersed in the dispersion or latex for a standard time, allowed todrain, and again put through the tworoll wringer. The polymer of thelatex was thereafter immediately gelled by immersing the impregnated webinto a 50% solution of acetic acid in methanol. The impregnated webswere then washed free of soap and acid with running water, and pressedfree of excess water. Thereafter, the webs were dried at a temperaturebelow 90 C. to prevent the polymeric binder from curing until the webswere placed between sheets of cellophane and Bristol board and pressedat 500 lbs. per sq. in. pressure and at a temperature above the flowtemperature of the particular extensible or hinder polymer employed.

All of the compositions tabulated in Table l were pre- In stretching theinitial substantially vapor-impermeable sheets of the present inventionto improve vapor permeability, it was discovered that other changes inthe general physical properties of the resulting sheet were 9 produced.To investigate these changes, a series of initial composite sheets wasprepared, one series employing a butadiene/acrylonitrile copolymer asthe binder or extensible material, and another series employing aneoprene binder. All of these were prepared in a manner similar to theprocedure described hereinbefore; and, in all cases, crimped staplefibers of polyhexamethylene adipamide, 2 /2 in length and 3denier/filament, were employed as the relatively non-extensiblematerial. These initial sheets were then stretched from 10% to 50% inone direction or in both directions and then relaxed. After thisstretching step, the sheets recovered almost completely their originaldimensions except for thickness which increased. For example, after a50% stretch in two directions, there was almost a 30% increase inthickness.

TABLEZ Change of physical properties with stretchzng Unit Thick- Ten-Elonga- Tinius- .P. V., Example Binder Percent Percent Weight, uess,acity, tion, Modulus, Olsen gm. 100

Binder Stretch gJm inches p. s. i. percent p. s. i. Tear, rn. r.

grams 27 Butadiene/Acrylonitrile copolymer 52.2 10% 1D 762 .033 1,893195 6,894 18,450 69 d 51.0 20% 1D '742 .030 1, 456 192 3, 169 19, 300752 53. 0 30% 1D 687 .038 4, 221 95 2, 996 2, 250 3, 870

1D =one direction.

2D two directions.

Table 3 contains the results .of a test which may be employed for thepurpose of selecting the most desirable combinations of extensible andrelatively non-extensible materials from the adhesion standpoint.Assuming that the forces required to break the adhesive bonds areavailable, the ultimate limit would be the point at which the adhesionis greater than the breaking strength of the relatively non-extensiblefiber. On the other hand, in the case of employing a relativelynon-extensible material in the form of particles, the ultimate limitwould be represented by a situation wherein the adhesion is greater thanthe breaking strength of the extensible material.

To obtain a measure of the relative adhesion between various extensibleand relatively non-extensible materials, sheets of nylon, i. e.,polyhexamethylene adipamide, film (the relatively non-extensiblematerial) were fastened together by means of selected relativelyextensible or hinder materials in the form of a glue seal. This was doneby placing a film of various samples of relatively extensible materialsbetween adjacent layers of nylon film and pressing the layers atsubstantially the same temperature and pressure conditions employed toprepare the initial composite sheets. Thereafter, the seals were pulledin a Tinius-Olsen machine to obtain the force necessary to separate thelayers of nylon film. Table 3 presents the results of these tests.

It is to be understood that the purpose of the adhesion test is toprovide a method of selecting or screening various combinations ofextensible and relatively nonextensible materials which exhibit littleor no adhesion under the conditions required to composite the componentsto form the initial sheet. For example, as shown in Table 3,polyethylene does not adhere to nylon film under the conditions employedto form a composite sheet of nylon fibers impregnated with polyethylene,e. g., inserting a mat of intertangled nylon fibers between adjacentfilms of polyethylene and pressing the composite at 125 C. and 500 p. s.i. The advantage of employing extensible and relatively non-extensiblecomponents which exhibit little or no adhesion is illustrated by thefact that the initial sheet composed of nylon fibers impregnated withpolyethylene was rendered highly permeable to water vapor by very mildstretching, i. e., hand working. On the other hand, N-methoxymethylpolyhexamethylene adipamide forms a very strong adhesive bond withpolyhexamethylene adipamide; and a composite composed ofpolyhexamethylene adipamide fibers impregnated with N-methoxymethylpolyhexamethylene adipamide must be subjected to relatively drastictreatment, e. g., swelling with hot water and stretching while hot, toeifect any improvement in water vapor permeability.

For the purpose of comparison, Table 3 also includes data on compositesheets fabricated from the materials tested. In all cases, nylon fibers,2 /z in length and 3 denier/filament, were impregnated with the polymersindicated in Table 3. The method of preparing the initial compositesheet was the same as described hereinbefore except in the case of nylonfibers impregnated with polyethylene. The initial composite sheet inthis case was formed by inserting a carded web of intertangled nylonfibers between adjacent films of polyethylene and then pressing thelayers at 125 C. and 500 p. s. i. to form a composite sheet.

As shown in Table 3, those combinations of materials exhibitingintermediate adhesion between that of polyethylene to nylon andN-methoxymethyl polyhexamethylene adipamide to nylon (polyhexamethyleneadipamide) show intermediate values of leather permeability whenstretched to the substantially same extent. As a general observation, itshould be mentioned that in situations where it is desired to employ aparticular combination of an extensible and a relatively non-extensiblematerial which form relatively strong adhesive bonds, the Work requiredto produce a certain vapor permeability by stretching may be decreasedby reducing the amount of the extensible material in the initialcomposition.

TABLE 3 Efiect of adhesion of various extensible materzals to nylon 1Heat Percent Seal Extensible LPV, gmsJ Extensible Material ValueMaterial in Percent sq.

(Adhe- Composite Stretch meters/hr.

sion) Sheet s) Polyisobutylene 39 57 30 8, 514 Polyethylene 0 45 11, 000Neoprene 735 197 40. 5 30 11,000 MOA/VA 526 53 40 1,700 MCA/AN 536 61 302, 000 F/VA 456 50 30 7, 000 DOD 669 40 50 7,000 nylon 1,416 50 1, 500

1 Nylon=polyhexamethylene adlpemide. 1 MCAIVA=methyl Cellosolveaerylate/vinyl acetate copolymer. 3 M OAlAN=methyl Oellosolveaorylate/acrylonitrile copolymer. 4 E IVA=ethy1enelvinyl acetatecopolymer. 5 DCD=modlfied dichlorobutadlene. Nylon=N-methoxymethylpolyhexamethylene adipamide. 7 Mild Stretch (Hand Worked). 8 Swelled inhot water and stretched two ways 50% while hot.

TABLE 4 particles are uniformly dispersed throughout the neoprenebinder, and it has been found that when such particles are present inthe initial composite sheet, less stretching is required to obtain agiven LPV than in the case of stretching a similar composition whichdoes not contain particles. I

As many widely different embodiments may be made without departing fromthe spirit and scope of this invention, it is to be understood that saidinvention is in no Variation of physical properties with content ofextensible material 1 HAND WORKED Unit Tenacity] Tinius- Percent Exten-Weight Thick- Unit Elonga- Olsen L. P. V., sible Material (gms./sq nessWeight, tion, Modulus Tear/Unit gins/100 (Neoprene) meter (inches) p. s.i./ percent Weight, mfl/hr.

gins/m. lbs/gm.

STRETCHED 30% (TWO DIRECTIONS) UNSTRETOHED 1 Neoprene used as theextensible material.

The process and synthetic leather compositions of the present inventionhave been specifically illustrated in the foregoing examples in whichnylon (polyhexamethylene adipamide) fibers have been employed as therelatively non-extensible or reinforcing material. It is to beunderstood, however, that the relatively non-extensible material, asdisclosed hereinbefore, may be in the form of particles. Furthermore,other types of natural and synthetic fibers may be employed such asother types of polyamides and interpolyamides, such as polyhexamethylenesebacamide, polycaproamide, and various interpolyamides as described andclaimed in U. S. Patent No. 2,285,009, polyethylene terephthalate,polyacrylonitrile, rayon and various natural fibers such as cotton andwool, and mixtures of two or more of the aforesaid fibers. Variouspolymeric materials may be employed as the relatively non-extensiblematerials in the form of particles of various sizes, such as particlesof synthetic polyamides, and other synthetic linear thermoplasticmaterials. Furthermore, particles of thermosetting resins such asurea-formaldehyde, phenol-formaldehyde, etc., may be employed inparticulate form. In addition, various inorganic materials may beemployed in particle form such as titanium dioxide, zinc oxide, talc,clay and various diatomaceous earths.

It is within the scope of the present invention to employ mixtures offibers and particles of a relatively nonextensible material. Forexample, a mat of intertangled nylon fibers may be thoroughlyimpregnated with a latex of neoprene having an amount of zinc oxideparticles dispersed therein (as much as 40% of the particles, based ontotal solids, may be used). The zinc oxide wise restricted except as setforth in the appended claims.

I claim:

1. A process for preparing permeable sheet material which comprises hotpressing fibrous material with a dry, thermoplastic, extensiblepolymeric binder material, said binder material being substantially moreextensible than said fibrous material and having a flow temperaturebelow the deformation temperature of the fibrous material andconstituting from about 30% to about 70% by weight of the total weightof fibrous and binder materials, at a temperature above the flowtemperature of the binder and below the deformation temperature of thefibrous material to form a substantially water vaporimpermeablecompacted sheet and stretching said compacted sheet from about 10% toabout 50% of its original dimension to form a water vapor-permeable,substantially liquid-resistant sheet.

2. The process of claim 1 wherein the said compacted sheet is stretchedin two directions from about 20% to about 50% of its originaldimensions.

3. A process for preparing permeable sheet material which comprises hotpressing a non-woven mat of fibers with a dry, thermoplastic,extensible, polymeric binder material, said binder material beingsubstantially more extensible than the fibers and having a fiowtemperature below the deformation temperature of the fibers andconstituting from about 30% to about 70% by weight of the total weightof fibers and binder, at a temperature above the fiow temperature of thebinder and below the deformation temperature of the fibers to form asubstantially water vapor-impermeable compacted sheet and stretchingsaid compacted sheet from about 10% to about 50% of its originaldimension to form a water vaporpermeable, substantially liquid-resistantsheet.

4. The process of claim 3 wherein the said compacted sheet is stretchedin two directions from about 20% to about 50% of its originaldimensions.

5. The process of claim 4 wherein the fibers comprise nylon.

6. The process which comprises impregnating a nonwoven mat of nylonstaple fibers with elastomeric binder material having a flow temperaturebelow the deformation temperature of nylon, said binder materialconstituting from about 40% to about 60% by weight of the total weightof fibers and binder material, drying said impregnated mat, hot pressingsaid dried, impregnated mat at a temperature above the flow temperatureof the binder and below the deformation temperature of nylon, whereby toform a compacted sheet, and thereafter stretching said compacted sheetequally in two directions from about 20% to about 50% of its originaldimensions to form a water vapor-permeable, substantiallyliquidresistant sheet.

References Cited in the file of this patent UNITED STATES PATENTS Nae rtFeb. 3, Meers July 31, 'Kenworthy July 22, Respess Dec. 23, Respess Apr.14, Clifford June 14, Schur Jan. 1, Read Mar. 24, Austin Nov. 17,Francis Dec. 29, Kopplin Apr. 3, Hawley Jan. 7, Raymond et a1. June 21,Slack et a1. Oct. 25, Shearer Sept. 9, Rand Mar. 10, Biefeld et a1 Mar.30,

FOREIGN PATENTS Great Britain Sept. 14,

1. A PROCESS FOR PREPARING PERMEABLE SHEET MATERIAL WHICH COMPRISES HOTPRESSING FIBROUS MATERIAL WITH A DRY, THERMOPLASTIC, EXTENSIBLEPOLYMERIC BINDER MATERIAL, SAID BINDER MATERIAL BEING SUBSTANTIALLY MOREEXTENSIBLE THAN SAID FIBROUS MATERIAL AND HAVING A FLOW TEMPERATUREBELOW THE DEFORMATION TEMPERATURE OF THE FIBROUS MATERIAL ANDCONSTITUTING FROM ABOUT 30% TO ABOUT 70% BY WEIGHT OF THE TOTAL WEIGHTOF FIBROUS AND BINDER MATERIALS, AT A TEMPERATURE ABOVE THE TEMPERATUREOF THE BINDER AND BELOW THE DEFORMATION TEMPERATURE OF THE FIBROUSMATERIAL TO FORM A SUBSTANTIALLY WATER VAPORIMPERMEABLE COMPACTED SHEETAND STRETCHING SAID COMPACTED SHEET FROM ABOUT 10% TO ABOUT 50% OF ITSORIGINAL DIMENSION TO FORM A WATER VAPOR-PERMEABLE, SUBSTANTIALLYLIQUID-RESISTANT SHEET.